NGC 5128 is the nearest large
elliptical galaxies to our sun. It is also the nearest of the giant radio
galaxies, possessing an active galactic nucleus (AGN) and optically one of the
most luminous galaxies in the sky. Among many other things NGC 5128 is also the
prototypical postmerger elliptical galaxy. Structural peculiarities including
the prominent rotating disk of stars and gas and the complex shell structure of
its halo point to a large scale merger within the last billion years. Its
proximity offers astronomers a unique opportunity for a detailed look into the
peculiar formation histories of these giant stellar agglomerations. Strikingly
apparent within the galaxy are two major components, a large elliptical
structure which generates about 90% of the luminosity and a massive disk of
stars, dust and gas. The odd appearance supports the premise that a large scale
merger of an elliptical galaxy with a large barred disk galaxy occurred to form
NGC 5128 within the last billion years. Tidal streams of young stars have also
been identified in the halo of NGC 5128 thought to have occurred by
cannibalization of a nearby gas rich dwarf galaxy some 300 million years ago,
long after the large scale merger. Elliptical galaxies are typically devoid of
gas, dust and young stars. The unexpected presence of gas rich disk elements
within the structure of NGC 5128 supports the current belief that accretions and
mergers of low mass galaxies may be an important agent in the evolution of
galaxy formation.

NGC 5128 has many similarities
to the giant elliptical galaxy M87 including a unique population of globular
clusters which likely tell the tale of its complex history. Scattered throughout
the halo of NGC 5128 are some 2000 globular clusters, more than 3 times the
total number of globular clusters within our local group of galaxies. A subset
of its globular clusters are more massive than any of the clusters in our local
group and appear to bridge the gap between the most massive star clusters and
the lowest mass galaxies. These massive globular clusters are also significantly
younger (5 billion years) than the galaxy itself. The findings suggest that many
of the clusters are not intrinsic to the galaxy but may actually be the nuclei
of tidally stripped dwarf galaxies devoured by the galaxy in its remote past.
The possibility that massive globular clusters are the nuclei of dwarf galaxies
may help explain the presence of black holes in some massive globular clusters
such as G1 in M31.

In 1949, NGC 5128 was found to
be a loud source of radio energy, in fact the loudest radio source in its region
of the sky (second overall to Cygnus A) , earning it the designation Centarus A.
As a radio galaxy it releases 1000 times the radio energy of the Milky Way in
the form of large bi-directional radio lobes that extend some 800,000 light
years into intergalactic space. The source of the radio emission is very
compact, about 10 light days across and is believed to be a supermassive black
hole in the galaxy's center with a total mass of 200 million to possibly one
billion suns. As a radio galaxy, NGC 5128 belongs to the subgroup of galaxies
called Active Galaxies, which include Quasars, Seyfert galaxies, Blazars and and
Radio Galaxies. Active galaxies are distinguished by their prodigious energy
output which cannot be explained by their stellar populations and must have
another source. Active galaxies have in common an "Active Galactic Nucleus (AGN)"
which is believed responsible for their prodigious energy output. Supermassive
black holes are almost certainly the central engines of Active Galactic Nuclei,
powering the enormous outflows of energy which characterize this subgroup of
bright galaxies. Like most active galaxies, NGC 5128 shows high velocity
bi-directional jet phenomenon arising from its nucleus. The jets represent
streams of multimillion degree plasma leaving the nucleus at extremely high
velocities. The streams of hot gas are thought to be released when matter is
accreted by the supermassive black hole at the galaxy's center.

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